Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft either directly drives the generator rotor or through the use of a gearbox. The hub, gearbox (if present), generator and other systems are usually mounted in a nacelle on top of a wind turbine tower.
During operation of a wind turbine, the tower structure may undergo undesired vibrations, i.e. oscillatory or repeating displacements in any direction (fore-aft vibrations, side-to-side or lateral vibrations, longitudinal vibrations, torsional vibrations, . . . ) of any amplitude and of any frequency (high or low, constant or varying). These vibrations may be caused by different factors, e.g. wind acting on the tower, blades passing along the tower and locally disturbing the wind flow, vibrations transmitted from the gearbox to the tower, rotor movements, nacelle imbalances, vibrations from the hub transmitted to the tower etc.
If a tower is subjected to vibrations during a prolonged period of time, fatigue damage may result. Fatigue damage may lead to a reduced life time of the wind turbine tower and/or its components. Furthermore, the danger exists that when vibrations cause resonance in the wind turbine tower, this can lead to a potentially dangerous increase of the vibrations. A further complicating factor is that the size of wind turbines (rotor, nacelle, tower, etc.) keeps increasing. Also, as towers become higher, the effect of vibrations becomes more critical.
In the case of fore-aft vibrations or oscillations, where the tower sways back and forth in the direction parallel to the wind force and the rotor axis (X axis, see FIG. 1), it is known to dampen vibrations at the tower primary bending mode frequency (1st tower mode) by controlling the pitch angle of the rotor blades collectively, to cause a thrust on the nacelle and provide positive aerodynamic damping of the tower. U.S. Pat. No. 4,420,692 for example discloses such a method.
However, these methods are not effective to dampen vibrations at higher bending mode frequencies (2nd or higher tower modes), because the thrust on the nacelle does not affect these modes.
In wind turbines installed offshore (either floatingly arranged or on a foundation in the sea bed), the substructure and its connection with the tower also undergo vibrations, the connection region being particularly subject to fatigue.
Furthermore, in offshore turbines it is becoming apparent that the 2nd tower mode causes more deformation on the lower part of the tower and the substructure than in onshore wind turbines, and may be more relevant than the 1st mode; and this may also be the case in onshore turbines with very high towers.
It would thus be desirable to reduce fatigue load on the tower of wind turbines, and also on the tower substructure in the case of offshore wind turbines.
More particularly, it would be desirable to provide means to at least partly dampen 2nd and/or higher modes in wind turbine towers and substructures, particularly of fore-aft vibrations in which the tower sways back and forth in the direction parallel to the wind force and the rotor axis.